The present invention relates generally to fluid heaters, and more particularly to a demand anticipation control system for a high-efficiency, ultra-pure deionized (UPDI) water heater.
Larger wafer sizes, smaller device geometry, and greater circuit density have driven the need for very accurate temperature control of fluids used to produce semiconductors. Heated UPDI water is one such fluid used in the manufacture of semiconductor devices. However, UPDI water is a corrosive liquid. Thus, equipment used for heating UPDI water must be capable of withstanding the corrosive effects of the UPDI water that flows therethrough.
In addition, it is critical that the equipment used to manufacture semiconductor devices be capable of performing specific tasks while not introducing contaminates into the manufacturing process. One such fluid heater that can withstand the corrosive effects of UPDI water and not introduce contaminants into the manufacturing process is described and claimed in U.S. patent application No. 09/006,112 filed Jan. 13, 1998 and titled "High Efficiency Ultra-Pure Fluid Heater". The referenced U.S. patent application has been assigned to the same Corporation who is to be the Assignee of the present invention, and is hereby incorporated by reference in its entirety.
Fluid heaters conventionally utilize a temperature control system to maintain the desired operating fluid temperature. A commonly available Proportional Integral Derivative (PID) controller is good at maintaining an accurate fluid temperature as long as the load (e.g. fluid flow through the heater) is steady state. To achieve a steady-state fluid flow, a high-flow bypass has been commonly used to allow a steady state flow of UPDI water through a heating system. In this control scheme, UPDI flows at a constant rate, and either is used at the process, or is dumped for possible reclaim.
Alternatively, when fluid demand is low, a fluid heater can be operated in a reduced or low-flow mode (to maintain water purity), and when a high-flow is required, the output flow bypasses the process until the output temperature stabilizes. With the increase in chemical costs, largely due to purity levels and the cost of disposal, these methods are no longer acceptable in the industry. Flow rate changes, and temperature set-point changes for specific process "recipes" are becoming the standard rather than the exception.
Accordingly, it has been considered desirable to develop a new and improved control system for fluid heater which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.